To decide whether an inventor is violating a patent, it is necessary to carefully examine the patent’s claims (most patents contain more than one of these terse statements of the scope of the invention) and compare the elements of each claim with the elements of the accused infringer’s device or process. If the elements of a patent claim match the elements of the device or process (called “reading on” or “teaching” the device or process), an infringement has occurred. Even if the claims don’t literally match the infringing device, it is possible that a court would find an infringement by applying what’s known as the “doctrine of equivalents,” that is, the invention in the patent and the allegedly infringing device or process are sufficiently equivalent in what they do and how they do it to warrant a finding of infringement.

For example, Steve invents a tennis racket with a score keeper embedded in the racket handle’s end. The invention is claimed as a tennis racket handle that combines grasping and score keeping functions. Steve receives a patent on this invention. Later, Megan invents and sells a tennis racket with a transparent handle that provides a more sophisticated score keeping device than Steve’s racket. Even though Megan’s invention improves on Steve’s invention in certain respects, it will most likely be held to be an infringement of Steve’s invention, for one of two reasons:

Megan’s invention teaches the same elements as those claimed in Steve’s patent (a tennis racket handle with two functions) or

When considering what it is and how it works, Megan’s invention is the substantial equivalent of Steve’s invention (the doctrine of equivalents).

Under new legislation effective November 29, 1999, if a case is brought to enforce a patent on a method or process invention, the defendant can escape liability if the defendant was using the invention more than one year prior to the patent application date.

A patent owner may enforce his patent by bringing a patent infringement action (lawsuit) in federal court against anyone who uses his invention without permission. If the lawsuit is successful, the court will take one of two approaches. It may issue a court order (called an injunction) preventing the infringer from any further use or sale of the infringing device, and award damages to the patent owner. Or, the court may work with the parties to hammer out an agreement under which the infringing party will pay the patent owner royalties in exchange for permission to use the infringing device.

Bringing a patent infringement action can be tricky, because it is possible for the alleged infringer to defend by proving to the court that the patent is really invalid (most often by showing that the PTO made a mistake in issuing the patent in the first place). In a substantial number of patent infringement cases, the patent is found invalid and the lawsuit dismissed, leaving the patent owner in a worse position than before the lawsuit.

Patent protection usually ends when the patent expires. For all utility patents filed before June 8, 1995, the patent term is 20 years from date of filing, or 17 years from date of issuance, whichever period is longer. For utility patents filed on or after June 8, 1995, the patent term is 20 years from the date of filing. For design patents, the period is 14 years from date of issuance. For plant patents, the period is 17 years from date of issuance.

A patent may expire if its owner fails to pay required maintenance fees. Usually this occurs because attempts to commercially exploit the underlying invention have failed and the patent owner chooses to not throw good money after bad.

Patent protection ends if a patent is found to be invalid. This may happen if someone shows that the patent application was insufficient or that the applicant committed fraud on the PTO, usually by lying or failing to disclose the applicant’s knowledge about prior art that would legally prevent issuance of the patent. A patent may also be invalidated if someone shows that the inventor engaged in illegal conduct when using the patent – such as conspiring with a patent licensee to exclude other companies from competing with them.

Once a patent has expired, the invention described by the patent falls into the public domain: it can be used by anyone without permission from the owner of the expired patent. The basic technologies underlying television and personal computers are good examples of valuable inventions that are no longer covered by in-force patents.

The fact that an invention is in the public domain does not mean that subsequent developments based on the original invention are also in the public domain. Rather, new inventions that improve public domain technology are constantly being conceived and patented. For example, televisions and personal computers that roll off today’s assembly lines employ many recent inventions that are covered by in-force patents.

The Life of an Invention

Although most inventors are concerned with the rights a patent grants during its monopoly or in-force period (from the date the patent issues until it expires), the law actually recognizes five “rights” periods in the life of an invention. These five periods are as follows:

Invention conceived but not yet documented. When an inventor conceives an invention, but hasn’t yet made any written, signed, dated, and witnessed record of it, the inventor has no rights whatsoever.

Invention documented but patent application not yet filed. After making a proper, signed, dated, and witnessed documentation of an invention, the inventor has valuable rights against any inventor who later conceives the same invention and applies for a patent. The invention may also be treated as a “trade secret” – that is, kept confidential. This gives the inventor the legal right to sue and recover damages against anyone who immorally learns of the invention – for example, through industrial spying.

Patent pending (patent application filed but not yet issued). During the patent pending period, including the one-year period after a provisional patent application is filed, the inventor’s rights are the same as in Period 2 above. A patent application gives an inventor no rights whatsoever – only the hope of a future monopoly that doesn’t begin until a patent issues. However, most companies that manufacture a product that is the subject of a pending patent application will mark the product “patent pending” in order to warn potential copiers that if they copy the product, they may have to stop later (and thus scrap all their molds and tooling) if and when a patent issues. The PTO must keep all patent applications preserved in secrecy. The patent pending period usually lasts from one to three years.

In-force patent (patent issued but hasn’t yet expired). After the patent issues, the patent owner can bring and maintain a lawsuit for patent infringement against anyone who makes, uses or sells the invention without permission. The patent’s in-force period lasts from the date it issues until it expires. Also, after the patent issues, it becomes a public record or publication that prevent block others from getting patents on the same or similar inventions – that is, it becomes “prior art” to anyone who files a subsequent patent application.

Patent Expired. After the patent expires, the patent owner has no further rights, although infringement suits can still be brought for any infringement that occurred during the patent’s in-force period as long as the suit is filed within the time required by law. An expired patent remains a valid “prior-art reference” forever.